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Erection of Skewed Bridges: Keys to anErection of Skewed Bridges: Keys to anEffective ProjectEffective Project
Bob Cisneros, P.E.
Chief EngineerHigh Steel Structures Inc.
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CASE STUDY 1 CASE STUDY 2 CASE STUDY 3
IntroductionIntroductionSR 0028 OVER CSX RR
Erector: Alvarez, Inc./High
Steel Joint effort
HSSI Job No. PA 3127
I-87 NB Connector over I-287 EB
Erector: Yonkers Contracting Co.
HSSI Job No. NY 01066
SR 0031 over PA Turnpike
Somerset Co., PA
Erector: High Steel Structures, Inc.
HSSI Job No. PA 01004
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SingleSingle--Span Skewed Structure. Each end twists inSpan Skewed Structure. Each end twists in
opposite directionsopposite directions
I-87 NB Connector over I-287 EB Erector: Yonkers Contracting Co.
Span: 265 ftSpan: 265 ft
Case Study 1Case Study 1
Restores to plumb at
application of dead loads
Skew: 65Skew: 65 oo (PA 25(PA 25 oo)) Web Layover:Web Layover: 22 : 120: 120(4(4 at full noat full no--load profile)load profile)
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LONG-SPAN, DEEP, HIGHLY SKEWED STRUCTURES TEND
TO UN-TWIST AS DEADLOAD DEFLECTIONS OCCUR.
I-87 NB Connector over I-287 EB Erector: Yonkers Contracting Co.WEB LAYOVER: 4: 10 ft (shored to no-load)
YELLOW = TWISTED AT ERECTION (STEEL DL PROFILE)BLUE = PLUMB UPON APPLICATION OF DEADLOADS
SHORED TO DEVELOP
ERECTED PROFILE
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Steel Dead Load
Skewed Bridge ErectionSkewed Bridge Erection
Source: AASHTO/NSBA Steel Bridge Collaboration G12.1-2003, Fig. 1.6.1.B
Guidelines for Design for Constructibility (see www.steelbridges.org)
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Skewed Bridge ErectionSkewed Bridge Erection
Complete Dead Load
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Rte 9 over Connecticut River(New Hampshire & Vermont)
Erector: Cianbro Corp.
Twist:
15mm (5/8+)
Twisted Arch Lateral Bracing
Camber:
65mm (2 1/2+)
LB1,2
LB
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Detailing and Fabrication forDetailing and Fabrication forSevere SkewSevere Skew
DETAILER CONFIRMS IF OWNER WANTS STRUCTURE PLUMB AT ERECTION OR
FINAL POSITIONDETAILED TO FINAL POSITION: DURING ERECTION, CROSSFRAMES FORCE/TWIST
THE GIRDER WEBS OUT OF PLUMB.
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GIRDER WEB LAYOVER IS COMPUTED PURELY AS A GEOMETRIC EFFECT, BASED ON:
Span, LSkew,
Dead Load Camber (especially, concrete DL), CONCGirder Depth, D
GIRDER END ROTATION IS COMPUTED.
Ref. NYSDOTs Elastomeric Bearing Design Manual (1979), p. 15:STEEL=( 4STEEL)/L ; similarly, CONC CONC
NEXT, BASED UPON ROTATION & SKEW, THE OUT-OF-PLAN LAYOVER OF THE WEBS IS COMPUTED
AT SUPPORTS (OPPOSITE SENSE AT ABUTMENTS). IF COMPUTED TO ACCOMMODATE SLAB DL:
L.O. = { SIN [ Tan
-1
(
CONC) ] x Depth }/Tan (Skew)
ASSUMPTIONS:
THE GIRDERS ARE FLEXIBLE ALONG THEIR SPANS, SO THE BRIDGE CROSS-SECTION ATDIAPHRAGM LINES RETAINS ITS INTEGRITY
SIMILAR TO THE CHICAGO TRUSS ERECTION METHOD, WHERE:
(TRUSS) MEMBERS, AS ERECTED UNDER A NO STRESS (OR PRACTICALLY SO)
CONDITION, MUST BE BENT AND FORCED TO FIT THE END CONNECTIONS, THUS
INTRODUCING AN INITIAL REVERSE SECONDARY STRESS WHICH WILL THEORETICALLYDISAPPEAR WHEN THE STRUCTURE ASSUMES THE LOADING FOR WHICH IT IS
CAMBERED . (AREMA Ch.15, Sect 9.3.2.7)
FOR A SIMPLE SPAN, SKEWED BRIDGE, DETAILING TO FINAL POSITION
CREATES A TWIST IN THE GIRDERS AT TIME OF ERECTION, WHICH
RELAXES UPON APPLICATION OF CONCRETE DEADLOADS (Like wringing a
towel).
DESIGNER MUST EITHER
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DESIGNER MUST EITHER :ENSURE SUFFICIENT TORSIONAL FLEXIBILITY TO PERMIT WEB-LAYOVER (TWIST), OR
INCREASE VERTICAL (X-X) AND/OR TORSIONAL (CURVED) STIFFNESS TO REDUCE DEFLECTIONS
Examples:Short-span, heavy through girdersClosely-spaced, deep girders
deck placement sequence can restrain relaxation (DL deflection)
Twisting of the cross section about the z axis is resisted
by St. Venant and warping torsion. Conditions of
geometry, restraints and loading will determine the
relative importance of the two types of torsion.
- FHWA Bridge Engineering Course Source: FHWA, Bridge Engineering (Vol I),NHI Course No. 13064, pp. 13-31 thu 13-38 (1994).
End CF Int CF
St. Venant-type is pure, uniform & un-restrained torsion.
When the cross section is restrained or prevented from
warping, warping torsion develops.
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Two EqualTwo Equal--span Skewed Structure. Each Span Twists inspan Skewed Structure. Each Span Twists in
Opposite Directions Centered About the Parallel PierOpposite Directions Centered About the Parallel Pier
SR 0031 over PA Turnpike Somerset Co., PA Erector: High Steel Structures Inc.
Spans:Spans:
161 ft each161 ft each
Skew:Skew:7070 oo (PA 20(PA 20 oo))
Web Layover:Web Layover:11 : 48: 48
Case Study 2Case Study 2
Restores to
plumb at
application of
dead loads
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< 300 ft SPAN, 60o + REASONABLE (65o + FOR 200 thru 250 ft) Possibly higher for shorter spans
Check bearing rotational limit
D
XY
3) Curved & skewedL = 110 ft
R=400 ft
XY=65o
2) Two span
continuousL = 160 ft
XY=70o
1) Long-span (single)L = 270 ft
XY=65
o
4) Skewed widening
L = 230 ft
XY=75o
SAMPLE
COMPUTATIONS
General, Practical Skew Limits:General, Practical Skew Limits:
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Fabricating and ErectingFabricating and ErectingSkewed StructuresSkewed Structures
FABRICATION:
Girders built as for
un-skewed structures, tono-load Laydown profile
Cross-frame dropsdetailed to reflect no-
load, steel DL or finalposition
ERECTION:
Girders erected toapproximate no-loadprofile.
Shop-assembled crossframes will normallyforce the required twistcondition
If knock-down (field-assembled strut)
cross frames, can beproblematic.
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Cross Frame ConnectionsCross Frame Connections
preferred (by fabricators) 20o maximum skew
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Note: In the intermediate cross frame detail shown,Dimension ANote: In the intermediate cross frame detail shown,Dimension A is theis the
drop (usually tabulated beneath the detail for similar cross fradrop (usually tabulated beneath the detail for similar cross frames.)mes.)
DROP
Sk d i b li d i llSk d ti t b li t d i ll
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Skewed connection parts can be complicated, especiallySkewed connection parts can be complicated, especially
when combined with knockwhen combined with knock--downdown crossframescrossframes..
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REFERENCEONLY
Erecting HighlyErecting Highly--Skewed StructuresSkewed Structures
TOWERS & TIETOWERS & TIE--DOWNS FACILITATE TWISTDOWNS FACILITATE TWIST
REFERENCEONLY
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Asymmetrical, continuous flared structureAsymmetrical, continuous flared structure
subsub--girders framed rigidly into main girders at narrow spacinggirders framed rigidly into main girders at narrow spacing
vertical sagvertical sag--curvecurve
radius: 1,000 ft+/radius: 1,000 ft+/-- (varies, chorded)(varies, chorded)
Spans: 130 ftSpans: 130 ft --130 ft130 ft
Case Study 3Case Study 3
Skew: 64Skew: 64 oo (PA 26(PA 26 oo), & varies), & varies Web Layover:Web Layover: 11 max : 4 ftmax : 4 ft
SR 0028 over CSX RR, Allegheny County, PA Erector: Alvarez, Inc./HSSI joint effort
Combining Curvature, Severe SkewCombining Curvature, Severe Skew& Stiffness Successfully.& Stiffness Successfully.
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WHERE RESTRAINT BY CLOSELY SPACEDDIAPHRAGMS WAS ANTICIPATED,
CONNECTIONS WERE LEFT SNUG TIGHT
UNTIL AFTER DECK POUR (UTILIZED O.S.H.
AT THE DIAPHRAGM PLY OF CONNECTION).
Snug-tight locations at time of pourWEB LAYOVER: 2 +/- MAX (varies)
SNUG-TIGHT
SIMPLE SPAN SUB-GIRDERS FRAMED INTOCONTINUOUS MAIN GIRDERS NEAR 0.6L2(MAX M+). DESIGNER BALANCED RELATIVE
STIFFNESSES OF ADJACENT GIRDERS VIA
3DFE MODEL, TO MINIMIZE DIFFERENTIAL
DEFLECTION.
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Sub-to-main girder connection Minor, localized mis-alignmentSub-girder SG1
Through effective communicationamong key parties on the project,
only two holes (shown below) failed
to come into full alignment.
RELATIVE DL DEFLECTIONS (4 MAXIMUM) BETWEEN THE
CLOSELY SPACED SUB-
GIRDERS AND MAIN GIRDERS.
SETTING STEELSETTING STEEL
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PADOT BRG CONST. TOLERANCE
SETTING STEELSETTING STEEL
STRUCTURE ERECTEDSETTING SUB-GIRDER
(FASCIA, SG1)
SETTING SPAN 1 (EASIER)
WEB-LAYOVER
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Robert A. Cisneros, P.E.Chief Engineer
1770 Hempstead Road P.O. Box 10008
Lancaster, PA 17605-0008
(717)293-4086 Fax (717)293-4061
[email protected] www.highsteel.com
Thank you for your attention.
Long Island Expwy o ver Cross-Island Parkway
NYSDOT Contract D258437
Contractor: Perini Corp.
QUESTIONS?
Curved, FCM Trapezoidal Box Girders
Radius = 325 ft (min.)
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